The present disclosure relates to a photovoltaic device useful for generating an electrical current upon exposure to wide spectrum light, such as sunlight. The materials described herein can be used in organic solar cells.
A photovoltaic device typically contains a layer of a photoactive material sandwiched between two electrodes (i.e. a cathode and an anode). The photoactive layer can absorb the energy in a photon emitted by radiation, such as sunlight. This photon energy creates an exciton, or bound electron-hole pair. Depending on the material, the electron and hole can travel a short distance (on the order of several nanometers) before spontaneous recombination occurs. The exciton can move to a junction where they can be separated, so that electrons are collected at one electrode and holes are collected at the other electrode. This allows current to flow through an external circuit.
Such light absorption and charge generation is limited in organic photovoltaic devices. Organic semiconducting materials arouse interest due to their low-cost potential, light weight, and ease of processing. However, the materials typically used in organic solar cells do not optimally match the solar spectrum, resulting in a large fraction of the light energy passing through the device being lost (i.e. not converted into electrical current) and low power conversion efficiency. With over half of the total solar irradiance residing in wavelengths above 650 nm, capturing longer wavelengths in this near infrared (NIR) range of from about 650 nm to about 1000 nm is desirable.
One highly studied group of materials is that of metallophthalocyanines, which are a small molecule containing a metal atom at the center of a cyclic molecule. Metallophthalocyanines generally have a high absorption coefficient (α>105 cm−1) and hole mobilities of around 10−3 cm2/V·sec. They typically have a Q-band peak in the red to near-infrared wavelengths. However, they also have a relatively narrow absorption profile.
It would be desirable to provide a photovoltaic device that can capture more of the light energy present in sunlight and generate greater amounts of electricity, increasing the power conversion efficiency of the device.